Link adaptation by means of adaptive modulation and coding enables robust and spectrally efficient transmission over time-varying channels in a wireless system. The basic idea behind it is to estimate the channel at a receiver and feed this estimate back to a transmitter. The latter adjusts its transmission in order to adapt the modulation scheme and the code rate to the channel characteristics. Modulation and coding schemes that do not adapt to fading conditions require a fixed link margin to support an acceptable performance when the channel quality is poor. These systems are then designed for the worst-case channel conditions, and result in an inefficient use of the channel. Thus adaptive modulation and coding schemes are appealing, since they can increase the average throughput, and reduce the required transmit power and bit error rate. See for example the following references: (1) J. K. Cavers, “Variable-Rate Transmission for Rayleigh Fading Channels”, IEEE Transactions on Communications, pp. 15-22, February 1972; (2) W. T. Webb and R. Steele, “Variable Rate QAM for Mobile Radio”, IEEE Transactions on Communications, pp. 2223-2230, July 1995; (3) B. Vucetic, “An Adaptive Coding Scheme for Time-Varying Channels”, IEEE Transactions on Communications, pp. 653-663, May 1991; and (4) K. M. Kamath and D. L. Goeckel, “Adaptive-Modulation Schemes for Minimum Outage Probability in Wireless Systems”, IEEE Transactions on Communications, pp. 1632-1635, October 2004.
Moreover, a radio channel is always subject to some degree of frequency selectivity, implying that the channel quality will vary in the frequency domain. This variation in frequency can be beneficial for a link adaptation scheme over the frequency axis for multi-carrier systems such as OFDM systems. With adaptive modulation and coding in the frequency domain, a higher-order modulation (e.g., 16QAM or 64QAM) together with a high code rate is appropriate for frequency intervals (e.g., subcarriers or groups of sub-carriers) experiencing advantageous channel conditions in the frequency domain, where QPSK modulation and low-rate coding are used for frequency intervals with poor radio link conditions.